The Hartland Landfill is the main solid-waste disposal facility for the Victoria area (known as the Capital Regional District) in British Columbia. Since 1985, approximately $25-million has been spent on capital works, environmental controls and general site improvements. The site currently receives about 135,000 tonnes of waste annually and has a fifty-year life expectancy.
Hartland is located in the bedrock highlands of the Gowland range and is primarily situated on a north-south trending bedrock saddle. The climate is “cool Mediterranean” with annual rainfall in the range of 1,100 to 1,150 millimetres. Temperatures range from a low average of 2C in January to a high average of 23C in July.
The landfill has two phases. Phase one is the original active area with 4 million cubic metres of waste over a 20-hectare area, filled to capacity by March 1997. Phase two has a 10.8-million cubic meter capacity and filling began April 1997. A design and construction program was undertaken in 1995 to manage the closure of phase one. (A small part remains open for disposal of controlled and special wastes.)
The selection of the type of final cover for phase one was based on the following factors: proven technology, cost, impermeability, availability of materials (particularly the clay barrier layer) and constructability. Test pads were constructed with a one-metre clay barrier layer.
Between 1993 and 1995 a final-cover test of seven systems helped to determine the following: the most appropriate construction methods and materials; the stability of the final cover on side slopes; the thickness of different layers; the most appropriate topsoil mix; and, the quantity of water infiltration through the barrier layer. Storms were simulated using a sprinkler system and the amount of infiltration was monitored. Vegetative growth and stability were also monitored.
Design & construction
The composite system incorporates both natural and synthetic materials that meet cost, impermeability, availability and constructability objectives. The design consists of (from the bottom up): a 300 mm drainage layer of crushed aggregate for the top of covered waste; a medium-weight geotextile; a 300 mm compacted clay secondary barrier; a 50 mm sand friction layer; a 40 mm PVC primary barrier; a 50 mm sand protection layer; a 300 mm of 25 mm minus crushed aggregate subsoil; and, a 300 mm of hydroseeded topsoil.
The closure of phase one took about four years; the entire project could not be completed in one dry season. Prior to the placement of any of the composite closure layers, the existing intermediate clay cover was proof-rolled to level out any uneven areas or soft spots that had developed during differential settlement. The contractor made two passes using a 10,000 kg vibratory grid roller and any soft spots (or depressions) were brought to grade with the drainage layer.
All aggregate used in the construction of the cover was produced from an onsite rock quarry operation except for the sand layers (which required a fine product). Excavation of the rock increased the landfill life expectancy and was cheaper than importing aggregates.
For the closure of the top of the landfill (where the slopes were much less than the 3.5:1 side-slopes) the sand friction layer was eliminated and the sand protection layer was replaced with a medium-weight non-woven geotextile to provide the required protection for the geomembrane.
The phase one landfill was constructed using alternating layers of waste and a clay cover so surface or groundwater within the refuse mass moves in a lateral direction and breaks out the side-slopes of the landfill. The closure system incorporates the drainage layer design to pick up any side-slope leachate breakouts and then conveys leachate to a toe drain. This layer also acts as a leveling layer, i.e., to bring up the grade in any areas where there may have been differential settlement. (The actual thickness varies between 300 mm and 450 mm.)
Approximately 50,000 m3 of imported clay with a hydraulic conductivity of 1 x 10-7cm/s was installed on non-woven geotextile to form the secondary barrier layer. The clay was placed in loose lifts (not exceeding 180 mm) and compacted to at least 95 per cent Standard Proctor Density by a 10,000 kg vibratory-grid roller compactor. Clay moisture was closely monitored and water was added to ensure optimum compaction. Full-time inspection was provided for quality control and assurance.
The main barrier layer is a 40 mm textured (one-side) PVC geomembrane sandwiched between a sand friction (25-50 mm) and protection layer (25-50 mm). The membrane was laid in sheets in the same direction of the slope and seamed together with a single or double hot-wedge fusion weld. Prior to placement of the sand protection layer, the membrane was anchored with sandbags. A stone slinger was used to place the sand so as to prevent undue travel on the unprotected membrane.
A continuous (or placed in one lift) 300 mm layer on 25 mm minus (aggregate material) subsoil — spread with a small bulldozer and left uncompacted — conveys clean water that percolates through the topsoil layer laterally to the runoff collector ditch with a minimum of head build-up. (See diagram.)
A topsoil layer of peat material — which is taken from the lake basin that now forms part of the phase two landfill — protects against erosion of the underlying layers and is a good medium for healthy vegetative growth. The peat was spread with a low-ground-pressure bulldozer working up the slope in one continuous 300 mm lift. When installation was complete, the slope was hydroseeded. (“Hydroseed” is a drought resistant mixture of grass seed, fertilizer, tackifier and wood fibre mulch that is mixed in a slurry with water and sprayed on the ground surface.)
The complete cost to install the system was $33 to $36 per square metre including the cover, leachate collection and surface water drainage and control systems. Generally, the cover is performing as expected. It has reduced overall leachate production, provided aesthetic and odour barriers and a tight cover to the landfill that enhances the overall performance of the gas extraction system.
Chris Riddell, C.Tech is superintendent of solid waste operations for the Hartland Landfill in the Capital Regional District, British Columbia.